ABSTRACT
The layered cobaltite Sr(2)CaYCo(4)O(10.5) with formal average cobalt oxidation state close to 3+ has been studied as functions of both temperature and pressure up to 4 GPa by neutron powder diffraction (NPD). The crystal structure is shown to have tetragonal symmetry (space group I4/mmm; 2a(p) × 2a(p) × 4a(p) superstructure), and the magnetic structure at ambient pressure is found to be G-type antiferromagnetic with TN close to 310 K. The magnetic moments within the CoO(6) octahedral layers and anion-deficient CoO(4.5) layers are 1.2µ(B) and 2.8µ(B), respectively. At 25 K, and applied pressure of 3.5 GPa is sufficient to completely suppress a long-range magnetic order. This result is interpreted in terms of a pressure-induced high-to-low spin state crossover of the Co(3+) ions.
ABSTRACT
La0.7Sr0.3Mn(3+)0.85Sb(5+)0.15O3 and La0.7Sr0.3Mn(3+)0.8Sb(5+)0.1Ge(4+)0.1O3 compounds with dominantly isovalent Mn3+ ions were studied by neutron powder diffraction and magnetization measurements. The compounds are basically ferromagnetic, with magnetic moments slightly above of 3 µB/Mn. Upon temperature decrease, the compounds exhibit structural transition from a rhombohedral phase to orbitally disordered orthorhombic one. The structural transitions occur well above the temperature of magnetic ordering (Tc ≈ 130 K). It is suggested that the ferromagnetic state is governed by the positive part of superexchange interactions Mn(3+)âOâMn(3+), which is enhanced by Mn(eg)âO(2p) hybridization.
ABSTRACT
The evolution of the crystal structure and the magnetic properties was investigated in the La0.5Sr0.5CoO(3-δ) (0 < δ < 0.3) system as a function of the oxygen deficit δ. Compounds with a low oxygen deficit (δ < 0.1) are shown to be predominantly ferromagnetic, while further increase (δ > 0.1) gradually changes the magnetic structure from ferromagnetic to G-type antiferromagnetic and causes a structural transition from rhombohedral to cubic symmetry. Resistivity and magnetoresistance at low temperature increase with increasing of oxygen vacancies. It is argued that oxygen reduction facilitates stabilization of the high spin state of Co(3+) ions. Antiferromagnetic interactions between cobalt ions in the high spin state are found to dominate in compounds with the oxygen deficit δ > 0.18.
ABSTRACT
Neutron powder diffraction and magnetization measurements of the Bi(1-x)Ca(x)FeO3 (0.05 ≤ x ≤ 0.14) compounds were carried out to follow the effect of the heterovalent A-site doping on the long-range structure and magnetic properties of the BiFeO3 multiferroic. Ca substitution induces the appearance of weak ferromagnetism in the initial ferroelectric R3c phase, but modifies the picture of polar displacements, so the average PbZrO3-like antiferroelectric structure is stabilized at x = 0.11. Further increase of the Ca content leads to transformation of the antipolar ionic shifts to give rise to the Pbam â Imma transition near x = 0.14. A structural study performed for the x = 0.05 compound at high temperature revealed the R3c â Pnma phase transition at 950 K. For x = 0.1 samples, an intermediate heating-induced structure separating the R3c and Pnma phases was found.
ABSTRACT
From magnetic, specific heat, (170)Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb(2)Sn(2)O(7), we show that below the first order transition at 0.15 K all of the Yb(3+) ions are long-range magnetically ordered and each has a moment of 1.1 µ(B) which lies at ≃ 10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.
ABSTRACT
The magnetic properties together with the crystal and magnetic structures of the cobaltites La(1-x)Ba(x)CoO(3) (for x = 0.2 and 0.3) are determined by DC magnetization, AC magnetic susceptibility and neutron powder diffraction measurements over a broad spectrum of temperatures. For x = 0.3 a rhombohedral structure with space group [Formula: see text] is maintained at all temperatures below 300 K. On the other hand, for x = 0.2 the refinement of the neutron data below 150 K indicates the coexistence of two structures, [Formula: see text] (rhombohedral) and Pbnm (orthorhombic), respectively, in a ratio of â¼48/52. Both compounds (x = 0.2 and 0.3) show a ferromagnetic long range order. The data fit well with the Co(3+) ions in the intermediate spin state and the Co(4+) ions in a low spin state.
ABSTRACT
Neutron powder diffraction studies of the structural and magnetic properties of the La(1-x)Sr(x)Nb(y)Co(1-y)O(3) solid solutions (x = 0.2, 0.5; y = 0, 0.075, 0.1) have been performed. Substitution of Co(2+) by Nb(5+) prevents the formation of Co(4+) and leads to stabilization of the Co(3+) ions in the high- or intermediate-spin state. This is accompanied by the significant change of the Co-O bond length as well as Co-O-Co bond angle in the CoO(6) octahedron. The obtained data are in agreement with the negative sign of the magnetic exchange interactions Co(3+)-O-Co(3+) in a relatively wide range of the Co-O-Co bond angles. Diamagnetic dilution by the niobium ions prevents long-range magnetic ordering and strongly increases magnetoresistance at low temperature.
